A swimming pool, museums, and even tracks for running and go-kart racing: hundreds of buildings are located under Helsinki, forming a second underground capital. These spaces double as shelters in the event of war. Initially planned and constructed during the Cold War, this network is still expanding. There are 10 million m3 of underground spaces (parking, sports, the metro, etc.), more than 400 premises, 220 km of technical tunnels (for supply traffic and waste management), 24 km of raw water tunnels and 60 km of utility tunnels. This development is controlled by the Helsinki Underground Masterplan, a 3D plan that harmonises development on and below the surface – a globally unique instrument for sensible and sustainable urban expansion. The raw condition of bedrock is a hotbed for experimentation: a matter charged with speculative potential for future ecologies, adaptive reuse of shelters and urban climatology. The case of Helsinki gives us an idea about how the diversity of carved underground spaces subtly shapes social, architectural and local identities.